All laboratory strains of mice contain endogenous mouse mammary tumor virus (MMTV) DNA sequences in their genome. However, different strains of mice differ drastically in their tumor incidence (from less than 1 to 100% at 18 months) and their mode of transmission (milk vs gametes). The objective of this project is to relate differences in expression of mouse mammary tumor virus with differences in the virus specific nucleic acids present in normal and mammary tumor tissue and with the relative arrangement of the MMTV specific DNA sequences in the host cell genome. The proposed work involves the mapping of the endogenous and exogenous MMTV present in various mouse strains and the examination of MMTV specific DNA sequences in hormone dependent and independent mammary tumors. This work will involve nucleic acid annealing, restriction endonucleases and the DNA transfer procedure, and somatic cell genetics.